In Jurassic Park, the huge sauropods were pictured grazing on the tops of tall trees. This would have been physically implausible, said an Australian biologist in a letter to Science.1 A brachiosaur’s head would be 9 meters above its chest. That would require 750 mm (Hg) of blood pressure. The problem of circulation, not only of hypertension, would have made these beasts keep their heads down, he said.
This approach to the problem does not rely on knowledge of heart size or strength, but involves an estimate of the energy cost of the circulation, based on two axiomatic relationships between metabolic rate, blood flow rate, and blood pressure. First, the Fick Principle states that an animal’s aerobic metabolic rate is proportional to blood flow rate. Second, the rate of work done by the left ventricle is proportional to the product of blood flow rate and mean arterial blood pressure. Therefore, cardiac work is proportional to the product of metabolic rate and blood pressure. Cardiac work averages about 10% of the metabolic rate in mammals that have a mean arterial blood pressure of about 100 mm Hg. An animal that produces 750 mm Hg would consequently have a cardiac work rate 7.5 times higher. Its metabolic rate would increase to 165%, and it would expend 45% of its total energy requirements just to circulate the blood. These percentages would be the same whether the animals were active or resting, high-energy endotherms or low-energy ectotherms. The high cost of high browsing makes it energetically more reasonable to keep the head down and move the neck horizontally rather than vertically….
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